/Engine/Source/Runtime/Slate/Public/Framework/Application/SWindowTitleBar.h
This is an advanced method, only for fancy windows that want to override the look of the title area by arranging those widgets itself.
virtual void MakeTitleBarContentWidgets ( [TSharedPtr](API\Runtime\Core\Templates\TSharedPtr)< [SWidget](API\Runtime\SlateCore\Widgets\SWidget) > & OutLeftContent, [TSharedPtr](API\Runtime\Core\Templates\TSharedPtr)< [SWidget](API\Runtime\SlateCore\Widgets\SWidget) > & OutRightContent )